A turbofan propulsion engine for aircraft includes a gas turbine engine core that powers a turbofan. The air moved by the fan flows in a bypass air duct formed annularly around the engine core. A nacelle system includes all the structure that defines the bypass air duct, the inlet to the fan and engine, and the exhaust nozzles for the bypass air in the bypass air duct and the exhaust from the core engine. A schematic representation of typical nacelle components is shown in FIG. 1.
The inside surface of the fan duct is formed by structure typically termed the inner fixed structure by those skilled in this art. The inner fixed structure (IFS) is typically fashioned in two halves which join together, one of the halves of a typical IFS is shown in FIG. 2. The inside surface of the IFS faces the engine core and helps define a chamber around the engine core to protect it and to separate any fire which might break out around the engine core from escaping and spreading to other areas of the nacelle. The outer surface of the IFS faces the bypass air duct and must be smooth to minimize drag. The inside surface of the IFS may be exposed to extremely high temperatures radiating from the operating engine core and must be capable of withstanding those temperatures while remaining structurally sound. The outside surface of the IFS is very effectively cooled by the fast flow of ambient air in the bypass air duct. This means that a wide temperature gradient typically exists between the inside and outside surface of the IFS, which can present many challenges that are addressed through material selection and design.
To minimize the temperature gradient, and to minimize the maximum temperature that the IFS material will be exposed to, it is common to cover the inside surface of the IFS with a protective heat blanket. The protective heat blanket is a separately formed component which is attached to discreet locations to the inside surface of the IFS. The protective blankets are fashioned as thin, flexible sheets with outer skins of titanium or stainless steel or other heat resistant metal. The sheets can be molded around the complexly curved surfaces of the IFS and around components, wires or tubes protruding through. The heat blankets, sometimes along with designed cooling flows of air from the bypass air duct, are effective enough in reducing the temperature of the inner surface of the IFS such that the IFS can typically be made from a carbon fiber reinforced composite material, which is a desirable material for its light weight.
Heat blankets themselves, however, add weight and size to the nacelle. The heat blanket may require the inner surface of the IFS to be radially spaced up to an inch or more further from the engine core case. This extra inch means that the overall nacelle system will be larger in diameter, which can significantly add to the weight and drag of the nacelle. Also, the heat blankets are expensive, add extra time for installation, are sometimes not as reliable as desired, and are difficult to work with when removing or repairing any portion of the IFS during service.
It is desired to have an IFS which does not require heat blankets, but remains lightweight and meets the structural and temperature requirements.